The invention relates to methods and devices for installing discontinuous sheaths on cables, that is to say sheaths composed of identical tubular sections following on axially one after the other, as well as to the sheathed cables obtained.
It is aimed more particularly at the case where the cables in question are the cores of oblique stays and are each composed of a plurality of strands or elements which are themselves formed individually by wires or by assemblies of wires which are parallel or twisted, the said stays serving especially to equip cable-stayed bridges, connecting the towers of these bridges to their deck.
The invention is aimed more particularly still, but not exclusively, at the case where, in order to reduce the windage of each stay, its sheath has a cylindrical external profile of circular cross-section, as was described in Patent FR-A-90/04180.
The thermal expansion coefficient of the sheaths in question is generally different from that of the cables surrounded by these sheaths: in the preferred case where the sheath is formed from a plastic (such as polypropylene or high-density polyethylene), the difference between these two coefficients is considerable, that of the sheath being much higher than that of the cable.
To accommodate the thermal strains of the various sheath sections along the oblique cable which they surround, it has been proposed to provide mutual over-lapping of the contiguous ends of these sections, the lower end of each section being for this purpose widened out in the form of a bell or tulip so as to surround the upper contiguous end of the section immediately below, this accommodating the mutual sliding movements of these two ends.
It is advisable that the mutual sliding movements in question be prevented from occurring non-uniformly along the stay and from resulting in a complete disengagement of some ends of sections out of the adjacent ends and consequently leading to the thus unsheathed portions of the stay being directly exposed to the open air.
Such non-uniform sliding movements are inevitable if the sheath sections are not individually attached to the cable at their initial fitting positions: thus, under this assumption of the absence of an individual attachment, the weight of each section, due to the obliquity of the cable, tends to lower it, with sliding movements of its lower opened-out end against the overlapped upper end of the section below, and this will continue until the bottom of the said opened-out end butts up against the edge of the said overlapped end.
The disengagements resulting from such uncontrolled sliding movements have several drawbacks and, in particular, the following:
the unsheathed portions of the stay have a high windage, PA1 the discontinuities which result from this are unsightly, in particular because of the fact that the strands constituting the cable are generally covered with black jackets and that the sections of external sheath are, on the contrary, generally light-coloured (grey or white), PA1 the cumulative displacements and strains of the various sheath sections, combined with gravitational attraction, may lead in very cold weather to these sections collecting at relatively low positions, this requiring, as a result of the subsequent expansion of the sections due to them warming up thereafter, sliding movements of relatively large magnitude of the upper sections along the cable, sliding movements which may lead to blockages and even to local ruptures, or the very least to frictional wear. PA1 each fastener comprises first fastening means permanently fastened to the cablet and second fastening means suitable for removably interacting with the first ones and with a portion, preferably provided for this purpose, of a sheath section so as to link these first means to this section, PA1 in a cable sheathed according to the preceding paragraph, the first fastening means comprise a base fastened to the cablet, a pin projecting from this base and a small elongate plate mounted so as to rotate at the end of this pin and the second fastening means comprise an oblong opening hollowed out longitudinally in the sheath section, the said oblong opening being sized so as to be able to let the small plate pass when it is oriented longitudinally and not when it is oriented transversely, PA1 in a sheathed cable according to the paragraph which precedes the previous one, the first fastening means comprise a base fastened to the cablet and a stud projecting radially from this base and itself being pierced close to its free end by a transverse bore and the second fastening means comprise a hole hollowed out in the sheath section and a cotter pin suitable for interacting with the bore of the stud, on the outside of the sheath section, PA1 the fastener is formed by a U-bolt suitable for straddling the cablet, the two feet of this U-bolt being suitable for passing through two holes hollowed out in the sheathed section and for having two lock nuts placed on the end of them, on the outside of this section. PA1 an inextensible cablet at least as long as the cable is employed, PA1 around this cablet and the base of the cable are successively placed each of the sections intended to form the sheath, PA1 a given point on each section is fixed locally to the said cablet, the relative positions of the points for fixing the various sections to the cablet being identical and the distance between the fixing points, which follow on one after the other along the cablet, being slightly less than the minimum length envisageable for each section, PA1 and then the cablet is pulled taut along the cable by exerting a pulling force on its upper end in such a way that, at the end of pulling, the various sheath sections follow on uniformly one after the other along the cable with mutual overlapping.
In order to remedy these drawbacks, it has already been proposed to fasten one point on each sheath section locally to the cable after first installing it onto the cable.
In the embodiments which have been proposed hitherto for ensuring such local fixing, this fixing is performed directly on site, in particular by installing a metal collar around the upper end portion, of each sheath section, which is covered by the lower flared-out portion of the section immediately above, the said collar being tightened in order to fasten it as desired.
Such an operation is tricky and expensive because of the fact that it has to be performed on site, on the already-tensioned oblique cable, in particular when the length of the cable in question is great and exceeds one hundred meters.